This invention relates to an electrolytic cell for the electrochemical deposition of one of the metals copper, zinc, lead, nickel or cobalt from an aqueous electrolyte containing the metal in ionogenic form, wherein the electrolytic cell has a trough-like container with a bottom, with side walls, and with at least one inlet and at least one outlet for the electrolyte, wherein numerous plate-like electrodes are disposed in the container and are partly immersed in an electrolyte bath, and wherein at least one anode and at least one cathode are connected with a direct current source.
Electrolytic cells of this type are known and described e.g. in DE-A-2640801, US-A-5720867 and DE-A-19650228. These cells include a single or only a few supply lines for the electrolyte, and attempts are made at conducting the electrolyte in the container in the desired way. From US-A-5720867, openings in the side walls are known, an electrolyte circulation being established inside a cell by means of bipolar electrodes.
It is the object underlying the invention to develop an electrolytic cell which is suited for current densities of several hundred and also more than 1000 A/M2, and which can utilize the resulting vigorous formation of gas for conducting the electrolyte.
In the above-mentioned electrolytic cell, the object is solved in accordance with the invention in that the bottom of the container which is in contact with the electrolyte bath has numerous openings for the passage of electrolyte, that below the bottom there is disposed at least one distribution chamber for recirculated electrolyte, and that at least one of the side walls of the container has at least one recirculation chamber for recirculating electrolyte from the electrolyte bath into the distribution chamber, the upper portion of the recirculation chamber being connected with the electrolyte bath and the lower portion of the recirculation chamber communicating with the distribution chamber.
In the electrolytic cell in accordance with the invention, part of the electrolyte is constantly recirculated from the electrolyte bath via the recirculation chamber and the distribution chamber through the openings in the bottom of the cell into the bath and to the electrodes. This recirculation of electrolyte ensures that all electrode areas constantly intensively get in contact with the electrolyte, even if a vigorous formation of gas is inevitable at high current densities. In the recovery of copper, e.g. gaseous oxygen develops at the anodes, which oxygen moves upwards at the anode surfaces in the form of bubbles and escapes from the electrolyte bath. In the inventive cell, the formation of gas and the related mammoth pump effect are utilized to constantly draw electrolyte from the distribution chamber through the openings in the bottom into the electrolyte bath and thus effect a circulation of the electrolyte. The mammoth pump effect of the ascending gas is strong enough, so that an external pump for moving the electrolyte can be omitted. The electrolyte flowing upwards from the bottom of the cell prevents that at the surfaces of the electrodes a boundary layer too much depleted in electrolyte is formed.
The electrodes of the electrolytic cell may be monopolar or bipolar electrodes. Monopolar electrodes may for instance be formed by a simple sheet (e.g. of titanium). Details of the formation of cells with bipolar electrodes are known e.g. from US-A-5720867 and DE-A-19650228. In the electrolytic cell, current densities in the range from 200 to 2000 A/M2 are employed, and preferably the current density is at least about 1500 A/m2.
Advantageously, at least half the electrodes have openings for the passage of electrolyte in the area which is immersed in the electrolyte bath. These openings improve the flow of electrolyte through the electrolyte bath to the recirculation chamber and thereby facilitate the circulation of electrolyte. Mostly, all electrodes are provided with such flow openings. The recirculation chamber for the electrolyte is disposed on at least one of the side walls of the container such that there is a certain distance from the point where the fresh electrolyte is supplied to the container from the outside. One possibility is to dispose the recirculation chamber at that side wall of the container which is nearest to the electrolyte outlet. It is, however, also possible to dispose recirculation chambers at those side walls of the container on which the electrodes are supported. Another possibility is to provide three side walls of the container with recirculation chambers. The recirculation chambers may also constitute single lines or passages through which the electrolyte flows downwards from the electrolyte bath below the bottom to the distribution chamber.
The numerous openings in the bottom of the container, through which the electrolyte flows upwards from the distribution chamber into the electrolyte bath, may have all kinds of shapes. The openings may for instance be round, oval or slot-shaped. Usually, it is ensured that 1 to 20% of the surface of the bottom consists of openings, the bottom surface area being calculated as a whole and without deduction of the cross-sectional areas of the openings. Mostly, the openings make at least 3% of the bottom surface area. Due to the intensive circulation of the electrolyte in the electrolytic cell it is possible to design the surfaces of the electrodes hanging in the electrolyte bath rather large. In particular, it is no longer necessary to ensure a relatively large distance between the electrodes and the bottom of the cell, so that electrolyte can uniformly flow towards all electrodes. In the inventive cell, the lower edges of the electrodes can have a distance from the bottom of only 5 to 50 mm.